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Thursday, January 18, 2007

Skull suggests human-Neanderthal link

Anthropology: Humans continued to evolve significantly long after they were established in Europe, and interbred with Neanderthals as they settled across the continent, according to new research published this week in the Proceedings of the National Academy of Sciences (PNAS) USA.

Professor Joao Zilhao of the University of Bristol, Professor Erik Trinkaus of Washington University and colleagues in Europe compared the features of an early modern human cranium found in the Pestera cu Oase (the Cave with Bones) in southwestern Romania with other human samples from the period (the Late Pleistocene). Differences between the skulls suggest complex population dynamics as modern humans dispersed into Europe.

The different fragments of the reconstructed cranium - named Oase 2 - were found in a Late Pleistocene bone bed principally containing the remains of cave bears. They were recovered during a systematic excavation project directed by Professor Trinkaus and Professor Zilhao between 2003 and 2005.

Radiocarbon dating of the specimen produced only a minimum age (more than 35,000 years), but similarity in morphological traits with the Oase 1 human mandible - found in 2002 on the surface of the cave, adjacent to the excavation area, and dated to about 40,500 years ago - lead the team to conclude that the two fossils were the same age. These are the earliest modern human remains so far found in Europe and represent our best evidence of what the modern humans who first dispersed into Europe looked like.

By comparing it with other skulls, Professor Zilhao and colleagues found that Oase 2 had the same proportions as modern human crania and shared a number of modern human and/or non-Neanderthal features.

However, there were some important differences: apparently independent features that are, at best, unusual for a modern human. These included frontal flattening, a fairly large juxtamastoid eminence and exceptionally large upper molars with unusual size progression which are found principally among the Neanderthals.

Professor Zilhao said: "Such differences raise important questions about the evolutionary history of modern humans. They could be the result of evolutionary reversal or reflect incomplete palaeontological sampling of Middle Paleolithic human diversity.

"They could also reflect admixture with Neanderthal populations as modern humans spread through western Eurasia. This mixture would have resulted in both archaic traits retained from the Neanderthals and unique combinations of traits resulting from the blending of previously divergent gene pools.

"The ultimate resolution of these issues must await considerations of larger samples of European early modern humans and chronologically intervening specimens. But this fossil is a major addition to the growing body of fossil, genetic and archaeological evidence indicating significant levels of biological and cultural interaction between modern humans and the anatomically archaic populations (including the Neanderthals) they met along the way as they spread from Africa into Eurasia."

It is apparent that the Oase 2 cranium indicates there was significant modern human morphological evolution since the early Upper Paleolithic, the researchers conclude. Oase 2 is 'modern' in its abundance of derived modern human features, but it remains 'non-modern' in its complex constellation of archaic and modern features.

Source: University of Bristol UK PR "40,000-year-old skull shows both modern human and Neanderthal traits" 15 January 2007

Pestera cu Oase 2 and the cranial morphology of early modern EuropeansErik Trinkaus et al.

Published online before print January 16, 2007, 10.1073/pnas.0610538104

Abstract

Between 2003 and 2005, the Pestera cu Oase, Romania yielded a largely complete early modern human cranium, Oase 2, scattered on the surface of a Late Pleistocene hydraulically displaced bone bed containing principally the remains of Ursus spelaeus. Multiple lines of evidence indicate an age of {approx}40.5 thousand calendar years before the present ({approx}35 ka 14C B.P.). Morphological comparison of the adolescent Oase 2 cranium to relevant Late Pleistocene human samples documents a suite of derived modern human and/or non-Neandertal features, including absence of a supraorbital torus, subrectangular orbits, prominent canine fossae, narrow nasal aperture, level nasal floor, angled and anteriorly oriented zygomatic bones, a high neurocranium with prominent parietal bosses and marked sagittal parietal curvature, superiorly positioned temporal zygomatic root, vertical auditory porous, laterally bulbous mastoid processes, superiorly positioned posterior semicircular canal, absence of a nuchal torus and a suprainiac fossa, and a small occipital bun. However, these features are associated with an exceptionally flat frontal arc, a moderately large juxtamastoid eminence, extremely large molars that become progressively larger distally, complex occlusal morphology of the upper third molar, and relatively anteriorly positioned zygomatic arches. Moreover, the featureless occipital region and small mastoid process are at variance with the large facial skeleton and dentition. This unusual mosaic in Oase 2, some of which is paralleled in the Oase 1 mandible, indicates both complex population dynamics as modern humans dispersed into Europe and significant ongoing human evolution once modern humans were established within Europe.

The 2002 discovery of a robust modern human mandible in the Pestera cu Oase, southwestern Romania, provides evidence of early modern humans in the lower Danubian Corridor. Directly accelerator mass spectrometry radiocarbon (14C)-dated to 34,000-36,000 14C years B.P., the Oase 1 mandible is the oldest definite early modern human specimen in Europe and provides perspectives on the emergence and evolution of early modern humans in the northwestern Old World. The moderately long Oase 1 mandible exhibits a prominent tuber symphyseos and overall proportions that place it close to earlier Upper Paleolithic European specimens. Its symmetrical mandibular incisure, medially placed condyle, small superior medial pterygoid tubercle, mesial mental foramen, and narrow corpus place it closer to early modern humans among Late Pleistocene humans. However, its cross-sectional symphyseal orientation is intermediate between late archaic and early modern humans, the ramus is exceptionally wide, and the molars become progressively larger distally with exceptionally large third molars. The molar crowns lack derived Neandertal features but are otherwise morphologically undiagnostic. However, it has unilateral mandibular foramen lingular bridging, an apparently derived Neandertal feature. It therefore presents a mosaic of archaic, early modern human and possibly Neandertal morphological features, emphasizing both the complex population dynamics of modern human dispersal into Europe and the subsequent morphological evolution of European early modern humans.